Re: Mutex/Lock

James Kanze <>
Sat, 31 May 2008 03:02:50 -0700 (PDT)
On May 31, 10:39 am, Chris Forone <> wrote:


bool Exit()

   if (active)
     active = false;
     // wait for thread termination :-)))


if i write:

active = false, mutex.Release()

i think, the mutex is first released -> error

is this right?

No. There's a sequence point at the comma, so all side effects
of the preceding expression must be finished before any side
effects of the following occur.

I do wonder about your code, however. You really need to
recover the mutex before the end of the if---otherwise, you'll
release it twice. And how do you wait for thread termination:
with a join, or with some sort of global thread counter? In the
latter case, you'll need the mutex to read it as well.

I generally use something like:

        ScopedLock lock( terminateFlagMutex ) ;
        terminateRequested = true ;

        ScopedLock lock( threadCountMutex ) ;
        while ( threadCount != 0 ) {
            threadCountCondition.wait( lock ) ;

        requestTermination() ;
        waitForAllThreadsToTerminate() ;


        ScopedLock lock( threadCountMutex ) ;
        -- threadCount ;
        threadCountCondition.notify() ;

at the end of each thread. (Note that this does require some
care when starting threads, since a race condition can occur
there if you're not careful. In my case, it's not a problem,
because all of the threads are always started by the main
thread, which is also the only thread which will call
terminate(), but if other running threads might start a thread
while you're calling terminate, you'll have to add some
additional logic in thread start-up to avoid the race.)

James Kanze (GABI Software)
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